One significant problem with heat pumps is a possible system malfunction whereby the room thermostat in the room to be heated and/or cooled by the heat pump commands compressor operation so as to either heat or cool the space but the compressor either does not operate or, in some cases, cycles on and off. Another system malfunction is where the compressor is energized and running but is not compressing the refrigerant; this is exemplified by the compressor valve failures and/or the loss of refrigerant. There are no obvious indications of these faults to a person near the thermostat because the compressor is at a location remote from that of the thermostat. This, in turn, with many systems, can mean (when the thermostat is calling for heating of the building) that auxiliary electric resistance heating is automatically used to heat the building; i.e., a backup heating system; however, this usually results in a much higher cost of heating. Accordingly, various prior art schemes have been devised for attempting to detect whether or not the compressor is running, or is running without refrigerant in the system, but all of these prior art arrangements have one or more shortcomings. For example, one prior scheme is to use the pressure of the refrigerant at the discharge side of the compressor; however, this does not provide a reliable enough signal. Also, it has been proposed that the value or magnitude of the electric current and/or electric voltage energizing the motor driving the compressor be monitored; however, these schemes only indicate that the motor is being powered and do not confirm that the compressor is actually pumping refrigerant.
This invention is an improvement over the invention disclosed in the copending application of Dale A. Mueller and Stephen L. Serber, Ser. No. 954,266, filed Oct. 24, 1978, now abandoned. Briefly, the present invention provides superior results to those of said copending application in detecting compressor startup problems and for providing appropriate control functions. More specifically, the present invention provides a more reliable control for detecting a defective compressor operation at the time of start up. For example, a heat pump unit may be housed in an enclosure which retains heat therein generated by the heat pump unit per se, e.g. by crank-case heaters, by solar effects, etc; the "artificially" high temperatures of the enclosure in turn would escalate the temperature sensed by the compressor discharge temperature sensor so as to possibly erroneously signal that the compressor was operating correctly when in fact the opposite was true. Also, there are times when it is difficult to obtain a large enough, e.g. 50.degree. F., rise in the compressor discharge temperature in the nominal time period, e.g. 5 minutes, required by the system of said copending application.
Accordingly, an object of the present invention is to provide a new and significantly improved compressor fault detection system for a reverse cycle refrigeration system.